Step ring.

A simple metal ring used to adapt a filter to a lens when the two have different filter thread diameters.

For example, lets say you have a 58mm filter and you want to attach it to a 52mm lens. (that is, the lens has a filter diameter of Ø 52mm - not a focal length of 52mm) You could use a simple step ring to do it, which could be very convenient if you want to save money and camera bag space by buying only one set of filters.

There are two types of step rings - step up and step down. Step up rings are used for attaching larger filters to smaller lenses. Step down rings are used for attaching smaller filters to larger lenses.

There can be vignetting problems (darkening around the edges of a picture) caused by step rings, however. First, if the ring is very tall the additional distance of the filter from the lens may cause vignetting problems, particularly with wide-angle lenses. And second, adapting a small filter to a large lens with a step-down filter is rarely a good idea unless you dont mind the vignetting effects that are likely.

Finally step rings can interfere physically with many lens hoods, especially if a very large filter is used on a smaller lens. (77mm filter on a 52mm lens, for instance)

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